Keep Your Engine Properly Tuned
Fixing a car that is noticeably out of tune or has failed an emissions test can improve its gas mileage by an average of 4 percent, though results vary based on the kind of repair and how well it is done.

Fixing a serious maintenance problem, such as a faulty oxygen sensor, can improve your mileage by as much as 40 percent.

Fuel Economy Benefit: 4%
Equivalent Gasoline Savings: $0.13/gallon

Check & Replace Air Filters Regularly
Replacing a clogged air filter can improve your car's gas mileage by as much as 10 percent. Your car's air filter keeps impurities from damaging the inside of your engine. Not only will replacing a dirty air filter save gas, it will protect your engine.

Fuel Economy Benefit: up to 10%
 Equivalent Gasoline Savings: up to $0.31/gallon

Keep Tires Properly Inflated
You can improve your gas mileage by around 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.4 percent for every 1 psi drop in pressure of all four tires. Properly inflated tires are safer and last longer.

Fuel Economy Benefit: up to 3%
Equivalent Gasoline Savings: up to $0.09/gallon

Use the Recommended Grade of Motor Oil
You can improve your gas mileage by 1-2 percent by using the manufacturer's recommended grade of motor oil. For example, using 10W-30 motor oil in an engine designed to use 5W-30 can lower your gas mileage by 1-2 percent. Using 5W-30 in an engine designed for 5W-20 can lower your gas mileage by 1-1.5 percent. Also, look for motor oil that says "Energy Conserving" on the API performance symbol to be sure it contains friction-reducing additives.

Fuel Economy Benefit: 1-2%
Equivalent Gasoline Savings: $0.03-$0.06/gallon

Note: Cost savings are based on an assumed fuel price of $3.07/gallon. Estimates for fuel savings from vehicle maintenance, keeping tires properly inflated, and using the recommended grade of motor oil are based on studies and literature reviews performed by Energy and Environmental Analysis, Inc., Washington, DC.

Source: www.fueleconomy.gov